a biochemical reaction mediator/speed-upper (catalyst). Enzymes are usually proteins (e.g. DNA polymerase), sometimes protein/RNA complexes (e.g. ribosomes), and sometimes RNA alone (ribozymes) and there are lots of different enzymes with different jobs & specificities. They bind to specific substrate(s) and provide the optimal conditions in their “active site” for a reaction to occur. For example, they might hold 2 substrates in the perfect position for one to attack the other, and/or they might give and take protons from the substrate to make them more reactive.
They are “catalysts” because they speed up chemical reactions without being used up in the process. So, after they help out with some reaction, they can do it again and again and again. I say “help out” because the enzyme doesn’t really “do” anything – it can only help make something that was “possible,” “likely.” Kinda like the whole “you can lead a horse to water but you can’t make it drink” saying, an enzyme can bring reactants together but can’t make them react. Instead, enzymes lower the activation energy required to get the reaction going.
It’s the “E” in E + S <-> [ES] <-> P (where E is enzyme, S is substrate, and P is product)